1. Field of the Invention
This invention relates to processes of weather control or modification, and more particularly, to methods and apparatus for dispelling fog.
2. Description of Related Art
There is a need for a method of dispelling fog at definable sites, such as airports or racetracks, in order that events such as flight arrivals and departures at airports or racing programs at race tracks can occur as scheduled. Although there has been substantial effort directed to meeting this need, the methods that have been developed still have not sufficed, for reasons including environmental pollution and cost.
Fog is a weather condition in which moisture particles are suspended in saturated air near the ground at levels of between 0.1 and 0.5 grams per cubic meter. Control or dispersement of that fog requires the evaporation or removal of these suspended particles. Various fog-dissipation methods have been tried in the past.
Heating fog-laden air evaporates the suspended water particles by increasing the air temperature, adding heat of vaporization, and increasing the amount of moisture that the air can hold. This process creates thermals of warm air which rise from the site, circulating cool, fog-laden air into the site. Heating air to dissipate fog was used during World War II in Great Britain when airplane engines were run along the runway. Barrels of burning fuel oil were also used along runways to add heat to the air and evaporate the suspended water particles. Both of these concepts added air pollutants, had high operating costs, and did not accomplish the desired result unless operated continuously.
Helicopter downwash has been applied to clear fogs and clouds with small scale success, but it has not proved practical for large-scale operations.
Subcooling the air removes suspended liquid and vapor by cooling and collecting the moisture in suspension, dropping the air temperature, and condensing moisture vapor from the air. Air is subcooled using a mechanical cooling system which circulates a cold liquid through a coil. Both the latent and sensible heat are removed from the air as it is circulated over the coil. After the moisture and sensible heat have been removed, the cooled dried air is reheated to the surrounding temperature so that it may absorb the suspended moisture from the wet air in the discharge area of the fan system. This process is expensive due to the mechanical removal of both sensible and latent heat and the addition of sensible heat back to the air. Large quantities of high-cost, limited-supply electricity are used in this process. The initial cost and maintenance costs are also high.
Hydroscopic particles can be seeded from aircraft to evaporate fog droplets and drop the resultant dilute solution droplets to the ground. This method has been tested in many places in the world with small-scale success, but since the material is thrown away every time, the cost is high and environmental pollution becomes severe. Examples of U.S. Pat. Nos. involving use of chemicals to dispel fog include 2,934,275; 3,274,035; 3,378,201; 3,434,661; 3,608,810; 3,608,820; 3,730,432; 3,802,624; 3,899,129; 4,600,147; and 4,653,690. U.S. Pat. No. 2,934,275 discloses a process of dispelling fog by forming a mixture of an aqueous solution of chloride salts of calcium, magnesium or zinc with thickening agents of starches, sugars or proteins into a mist having particles smaller than 1/2 mm in diameter; forming a normally liquid chlorinated aliphatic hydrocarbon into a mist having particles smaller than 1/2 mm in diameter; and commingling the mist with the fog to be treated. Calcium chloride is a chemical desiccant. Chemical dessicants act as defoliates and are environmentally harmful to plant life, in practical effect prohibiting their utility as an airborne treatment.
Calcium chloride has been used to dry city gas; for example, see the Chemical Engineers Handbook, Third Edition, John H. Perry, Ph.D., Ed., McGraw-Hill Book Co., Inc., at topic "Drying of Gases," pp. 877-880.